Solve each system by using substitution

7. First solve for x or y for either equation.
I generally solve for y, but it doesn't matter.
rearranging equation one will be: y=3x-4
Then replace that equation for y in the second equation
x+5(3x-4)=-4

Then rearrange to find x.
x+15x-20=-4
16x-20=-4
16x=16
x=1

Do the same for problem 8.
y is already solved for you in the second equation.
so x+7x+4=4
8x+4=4
8x=0
x=0. 

I’d say letter a..